matlab题：可以怎么改?

matlab题：可以怎么改?一个质量为75kg的人在某一高度静止下落,空气阻力F与v的关系满足F=-k ,其中常数k=0.22kg/m,根据牛顿第二定律知,若令g=9.80m/ ,求下落过程中速度v(t)和位移x(t)的表达式,并画图.>> clear>> syms t v x>> v=dsolve('Dv=g-k*v^2/m','v(0)=0');>> x=int(v);>> m=75;>> g=9.8;>> k=0.22;>> t=0:0.5:30;>> v=subs(v);Error using ==> sym.subsElements of the substitution cell array must be of the same size.>>x=subs(x);Error using ==> sym.subsElements of the substitution cell array must be of the same size.>> plot(t,v)Error using ==> plotConversion to double from sym is not possible.>> plot(t,x)Error using ==> plotConversion to double from sym is not possible.